1. Field of the Invention
The present invention relates to a magnetic recording medium having excellent running durability, a good winding appearance, low friction, no substantial abrasion of the back coating layer and excellent electromagnetic conversion characteristics. More particularly, the present invention relates to a magnetic recording medium having its feature in the composition of the coating layer (back coating layer) provided on the surface opposite to the surface on which the magnetic recording layer is formed.
2. DESCRIPTION OF THE PRIOR ART
Recently, magnetic recording media have been widely used in the fields of audio and video recording, computers, magnetic discs, etc. Accordingly, the amount of information to be recorded on magnetic recording media increases year after year, and there is an increasing demand for the improvement of the recording density of magnetic recording media.
As one of the conditions required for a magnetic recording medium for high density recording, it has been suggested to improve the coercive force and to make the medium thinner both from the theoretical and experimental points of view. There have been various proposals with respect to the compositions of the binder (binding agent), inorganic filler or lubricant for the back coating layer (e.g. Japanese Examined Patent Publication No. 29769/1982). However, there have been various problems with respect to the running durability (the winding appearance, the abrasion of the back coating layer, or the susceptibility of the back coating layer to scars due to abrupt stopping of the running tape), the adhesion of the back coating layer or calender stains during the process for the preparation (dropouts caused by the abrasion of the back coating layer or the calender stains). Further, none of them is fully satisfactory with respect to the electromagnetic conversion characteristics.
In a recording system presently available in which a magnetic head is employed, a spacing loss between the tape and the head is represented by 54.6 d/.lambda.[dB] where d is a distance between the tape and the head, and .lambda. is a recording wavelength. As is evident from this formula, in a short wave recording system having a high recording density which has been highly in demand recent years for the reason of e.g. abundant information to be recorded, the rate of the decrease in the output attributable to the spacing is extremely great as compared with the long wave recording system. Accordingly, even a fine foreign matter deposited on the tape surface is likely to lead to a failure to detect a pulse which should be detected at the time of reading out the information written in the magnetic recording medium and thus is likely to be detected as a dropout. As the causes for the foreign matters attributable to the dropouts, there may be mentioned magnetic powder fallen off from the magnetic layer of the magnetic recording tape as a result of a deterioration of the magnetic layer due to the repeated exertion of stress, or particles abraded off from the substrate during the tape running or dusts which are electrostatically deposited on the substrate surface and then transferred to the magnetic layer surface. In order to prevent the deposition of foreign matters, there have been proposed a method wherein a coating composition prepared by kneading an inorganic filler such as carbon black or graphite with an organic binder, is coated on the rear side of the substrate opposite to the magnetic layer side of the magnetic recording tape, to provide an antistatic property, or a method wherein the abrasion of the substrate is minimized by making the substrate tough. Further, in the case where the magnetic layer is made of a thin metal film, the magnetic recording medium tends to curl since the magnetic layer is thin. The backing layer plays an important role also for the prevention of such curling. By these treatments, the tendency for the increase of the dropouts by the repeated running operation can substantially be controlled. However, the level of the control is not yet adequate, and the dropouts are required to be further reduced.